Vibration damped sandwich systems

ABSTRACT

A VIBRATION DAMPED SANDWICH SYSTEM COMPRISING TWO HARD PLATES AND INTERPOSED BETWEEN THE PLATES A VIBRATION DAMPING INTERLAYER COMPRISING A GRAFT POLYMER OF A MIXTURE OF 66% BY WEIGHT OF STYRENE AND 34% BY WEIGHT OF ACROYLONITRILE ON COPOLYMERS OF 10 TO 30% BY WEIGHT OF VINYL ACETATE AND 90 TO 70% BY WEIGHT OF ETHYLENE.

Jan. 5, 1.971

H. OBERST ET Al- VIBRATION DAMPED SANDWICH SYSTEMS Filed NOV. 13, 1968 2 Sheets-Sheet 1 O5 dcomb +80 +100 +120 +1l-0 +160+180 OC dcomb 0 2 INVENTOR HERMANN OBERV JOAC-HM EEiIfaT GUNTHER DUVE ALFRED SCHOMMFR BY /hwuh v YM ATTORNEYS `Ian. 5, 1971 H, OBERST ET AL 3,553,072

VIBRATTON DAMPRD SANDWICH SYSTEMS Filed Nov. 18, 190e' s Shmmwmz lllllllllllllllillllll* FIG. 2u

llllllllllllllllllllll* FIG. 2b

INVE NTOFV,

AT TOF? NEYCJ United States Patent U.S. Cl. 161-165 4 Claims ASTRACT OF THE DISCLOSURE A vibration damped sandwich system comprising two hard plates and interposed between the plates a vibration damping interlayer comprising a graft polymer of a mix- Vture of 66% by weight of styrene and 34% by weight of acroylonitrile on copolymers of to 30% by weight of vinyl acetate and 90 to 70% by weight of ethylene.

The present invention relates to vibration damped sandwich systems having interlayers made of a graft polymer of a mixture of styrene with acrylonitrile on copolymers of vinyl acetate and ethylene.

It is known from South African specification No. 5,269 that highly valuable vibration damping materials of a broad temperature band suitable for damping bending vibrations of metal sheet constructions can be prepared by copolymerizing monomers whose homopolymers differ in their second order transition temperature by at least C. The above specification also reports that as vibration damping materials having a broad temperature band there can be used, among others, predominantly amorphous copolymers of ethylene and vinyl esters of fatty acids with 2 or 3 carbon atoms, for example vinyl acetate/ ethylene copolymers. The ethylene/vinyl acetate copolymers used up to now are, however, not satisfactory in all respects.

It has now been found that graft polymers of a mixture 0f 66% by weight of styrene and 34% by weight of acrylonitrile on copolymers of vinyl acetate and ethylene have outstanding damping properties and a very broad temperature range of damping and are, therefore, especially suitable for the vibration damping of sandwich Systems of hard plates, in particular metal sheets. Suitable copolymers of vinyl acetate and ethylene are preferably those containing about 10 to 30% by Weight of vinyl acetate, especially about 18% by Weight of vinyl acetate and, consequently, about 90 to 70% by weight, especially about 72% by weight of ethylene. From the economical point of view they have the advantage of being rather cheap.

The graft polymers of a mixture of 66% by weight of styrene and 34% by Weight of acrylonitrile on copolymers of vinyl acetate and ethylene are obtained by preparing a gel of the specified copolymer in a mixture of 66% by weight of styrene and 34% by weight of acrylonitrile containing a catalyst and performing a free-radical initiated polymerization at a temperature in the range of from about 60 to about 180 C. Especially good results are obtained with graft polymers of 35 to 65% by weight of a mixture of 66% by weight of styrene and 34% by Weight of acrylonitrile on 65 to 35% by weight of a copolymer as described above, for example a graft polymer of 60% by weight of a mixture of 66% by weight of styrene and 34% by weight of acrylonitrile on 40% by weight of a copolymer as specified above. As polymerization initiator there may be used tertiary butylhydro- 3,553,072 Patented Jan. 5, 1971 peroxide in the usual concentrations. With graft polymers of this type, the vibration damping effect of which critically depends on the Weight proportion of the monomers, very broad damping curves can be obtained having very high maximum damping values.

The present invention thus provides sandwich systems of hard plates, in particular metal sheets, having vibration damping, self-adherent interlayers consisting of graft polymers of a mixture of 66% by weight of styrene and 34% by weight of acrylonitrile on copolymers of vinyl acetate and ethylene, for which interlayers there are used graft polymers of 35 to 65 by weight of a mixture of 66% by weight of styrene and 34% by Weight of acrylonitrile on 65 to 35 by weight of a copolymer of 10 to 30% by weight of vinyl acetate and 90 to 70% by Weight of ethylene.

FIGS. la and 1b of the accompanying drawings are plots against temperature of the loss factor dcomb t0 illustrate the superior efficiency of the novel systems. The curve in FIG. 1a shows the loss factor dem, of a metal sheet arrangement of the invention as a function of temperature. For comparison, one of the most effective vibration damping materials known for metal sheet arrangements was used, namely a copolymer of 63% by Weight of vinyl acetate and 37% by weight of dibutyl maleate containing as plasticizer 15% by weight of 2-ethylhexyl phthalate and 15% by weight of tricresyl phosphate, calculated on the mixture (curve FIG. 1b). The copolymer of curve 1b was a thermoplastic adhesive especially suitable for producing vibration damped metal sheet sandwich systems comprising two outer metal sheets and a selfadherent thermoplast as damping interlayer. Systems of this type provide a damping effect which is extremely high in its maximum and cannot be exceeded for physical reasons (cf. H. Oberst and A. Schommer, Kunststoffe, vol. 55, p. 634 (1965), especially FIG. 9). In a symmetrical arrangement comprising two metal sheets, each having a thickness of 0.5 millimeter, and an interlayer 0.3 millimeter thick, the loss factor dcomb of the combined system, measured in the bending Wave method (cf. for example H. Oberst, L. Bohn and F. Linhardt, Kunststolfe, vol. 5l, p. 495 (1961)), almost reaches the value dcomb of 1. The known metal sheet damping by one-side damping coatings which are applied by spraying, trowelling or bonding in the form of layers of so-called damping vibration materials show loss factors generally of less than dcomb=0-2 with technically reasonable thicknesses or ratios of coating mass to plate mass of the combined system. With metal sheet sandwich systems which gain growing importance in recent times, it is possible to obtain damping values that are increased by a multiple, as shown by the present example, when the interlayer material has appropriate composition and thickness.

The temperature band width of the damping of the metal sheet sandwitch system does not only depend on the viscoelastic properties of the interlayer and the steel sheets but also to a considerable extent on the geometry of the arrangement, i.e. on the ratio of the layer thicknesses (cf. loc. cit (195), FIGS. 8 to 10). With metal sheet sandwich systems the band Width is advantageously defined as the range of the temperature interval Within which the value dcmb=0-05 is exceeded. The damping of metal sheets which are not damped by additional vibration damping ymeans in metal sheet constructions of various types corresponds to values dcombOl. The reference value dwmb of 0.05 thus means a considerable increase in the damping effect (by about 15 db (decibel)) as compared to the nil damping dcomb=0.0l.

In the curve shown in FIG. 1b the reference value dcomb of 0.05 is exceed in the mainly interesting frequency range of from to 1,000 c.p.s. (Hz.) at temperatures ranging from about 0 to 50 C. The temperature band width thus corresponds to about 50 C. Sandwich systems of this type are suitable for many technical elds of application. By modifying the content of plasticizer, it is possible to shift the temperature band of a high damping eiect to higher temperatures and thus to adapt the material to special technical uses, for example in machine units operating at elevated temperatures. This example of a metal sheet sandwitch system comprising a selfadherent interlayer having optimum properties of a vibration damping material with a broad temperature band prepared by copolymerizing appropriate monomeric compounds has hitherto not been surpassed by other arrangements of similar kind and may be taken as standard for judging the acoustic eiciency of the system according to the invention.

FIGS. 1a and 1b show the temperature curves of the loss factor dcomb of metal sheet sandwich systems comprising steel sheets of a thickness of 0.5 millimeter each and damping interlayers of a thickness of about 0.75 and 0.30 millimeter respectively, (FIG. 1b), for a frequency of 100 c.p.s. and 1,000 c.p.s.

The curves were measured with sandwich systems the interlayers of which consisted of (1a) a graft polymer of 60% by weight of a mixture of 66% by weight of styrene and 34% by weight of acrylonitrile on 40% lby weight of a copolymer of 18% by weight of vinyl acetate and 82% by weight of ethylene (according to the invention) (1b) a copolymer of 63% by weight of vinyl acetate and 37% of dibutyl maleate containing as plasticizer 15% by weight of 2-ethylhexyl phthalate and 15% by Weight of tricresyl phosphate, calculated on the mixture.

The arrangement la, whose monomer proportion lies in the optimum range has a surprisingly broad temperature band width with high maximum damping values of about 0.2 to about 0.3. With the arrangement 1a the center of damping is at about 110 C. for 100 c.p.s. and about 60 C. for 1,000 c.p.s. The temperature band width is about 135 C. for 100 c.p.s. and about 100 C. for 1,000 c.p.s. In arrangement 1a, the slow decrease of the damping towards high and low temperatures and the resulting broad temperature range of damping are especially favorable. The extellent vibration damping properties are maintained within a temperature range of about 30 C. to about 145 C. As compared with the standard system 1b, arrangement 1a has an appreciably broader temperature band width and better vibration damping properties at a. temperature above 50 C., so that arrangements of this type are suitable for quite a number of applications at elevated temperature for example machine units and appliances operated at elevated temperature).

A special advantage of the vibration damping material of the present invention resides in the fact that it may be applied continuously during the mass production of the metal sheet sandwich systems.

For this purpose it can be applied (1) in the form of the finished graft polymer and (2) as a gel of the copolymer in the styrene/ acrylonitrile mixture to be grafted on, which contains the polymerization initiator. In the latter case the graft polymer is produced by subjecting the sandwitch system comprising the gel as interlayer to a thermal treatment at a temperature in the range of from about 60 to about 180 C. It is a thermoplastic adhesive Which may be applied to the metal sheets by trowelling, brushing or pouring at elevated temperature. The sandwich system may then be advantageously cooled under pressure between rollers. Except for degreasing the metal sheets do not require a preliminary treatment and further adhesive. The adhesion is very good.

The vibration damping material of the present invention has a good resistance to ow. The metal sheet sandwich system may, within broad limits, be processed as normal metal sheets, that is they may be creased, bent, shaped, Welded and riveted. In this manner laminated systems are obtained having a damping height and temperature range of damping which makes them well suitable for many applications, especially at high temperatures.

Minor amounts of fillers, for example for improving the electric conductivity (improvement of resistance Welding) may be incorporated in the vibration damping materials. In order not to affect the damping effect adversely it is advantageous to use less than 1% by weight, preferably less than 0.5% by weight of filler, calculated on the copolymer. Suitable llers are, for example, heavy spar, silicic acid, graphite and soot.

The metal sheet sandwich system suitably has a total thickness in the range of from 1 to 6 millimeters. The interlayers may have a thickness of 0.1 to 1 millimeter, preferably 0.2 to 0.5 millimeter. A maximum damping effect is obtained with symmetrical laminated systems. With an equal weight, asymmetrical laminated systems have, however, a higher stiffness in exure and strength. Asymmetrical laminated systems are, therefore, preferred for those applications which require a highstrength with respect to the weight. The ratio of the thickness of the outer plates or metal sheets is preferably within the range of from 1:1 to 1:4.

FIG. 2 of the accompanying drawings shows sandwich systems with symmetrical arrangement (a) and asymmetrical arrangement (b) in which the intelayer 2 is interposed between the two outer plates or metal sheets 1.

What is claimed is:

1. A vibration damped sandwich system comprising two hard plates and interposed between the plates a vibration damping interlayer comprising a graft polymer of 35 l to 65% by weight of a mixture of 66% by Weight of styrene and 34% by Weight of acrylonitrile on 65 to 35% by weight of a copolymer of 10 to 30% by weight of vinyl acetate and to 70% by weight of ethylene.

2. A vibration damped sandwich system as claimed in claim 1, wherein the hard plates are metal sheets.

3. A vibration damped sandwich system as claimed in claim 1 wherein the graft polymer of the interlayer contains up to 1% by weight of a ller, calculated on the graft polymer.

4. A vibration damped sandwich system as claimed in claim 1, wherein the ratio of the thicknesses of the plates lies in the range of from 1:1 to 1:4.

References Cited UNITED STATES PATENTS A3,218,373 11/1965 Salyer et al. 3,271,188 9/1966 Albert et al. 3,314,904 4/1967 Burkus. 3,355,516 1l/l967 Hardt et al. 3,399,103 8/1968 Salyer et al. 3,414,638 12/1968` Hardt et al. 3,433,724 '3/ 1969 Chapiro et al.

JOHN T. GOOLKASIAN, lPrimary Examiner G. W. MOXON II, Assistant Examiner U.S. C1. X.R. 

